02 April 2008
In this study we have investigated the presence of nerve fibers containing dopamine, thyrotropin-releasing hormone (TRH) and neuropeptide Y (NPY) in the pars intermedia of the crested newt and we have examined the possible effect of these neurohormones on the release of alpha-melanotropin (alpha-MSH) by neurointermediate lobes in vitro. By means of immunohistochemistry, we observed the presence of tyrosine hydroxylase (TH)-immunoreactive fibers in the pars intermedia of the crested newt. Using a specific antiserum to dopamine, these fibers appeared to be mainly dopaminergic in nature. Unlike anurans, urodele amphibians do not exhibit TRH or NPY-like immunoreactivity in the pars intermedia. A perifusion system technique for newt pituitaries was developed to investigate the effect of dopamine, TRH and NPY on alpha-MSH secretion. Administration of increasing concentrations of dopamine (from 10<sup>–9</sup> to 10<sup>–5</sup> M) induced a dose-related inhibition of alpha-MSH release. This inhibitory effect was mimicked by the dopamine agonist apomorphine (10–<sup>6</sup>M). In contrast, the secretory activity of the newt pars intermedia was not affected by administration of synthetic TRH or NPY (up to 10<sup>–7</sup> and 10<sup>–6</sup> M, respectively). These results indicate that the neurotransmitter dopamine likely plays a pivotal role in the regulation of melanotropin secretion in urodele amphibians.